"Front Matter". In: Organosilanes in Radical Chemistry - Index of

40 Hydrogen Donor Abilities of Silicon Hydrides
hn or D
t-BuOOBu-t ƒƒƒ! 2 t-BuO: (3:13)
t-BuO: þ R3SiH !t-BuOH þ R3Si: (3:14)
Rate constants for Reaction (3.14) were measured directly by LFP techniques
and are collected in Table 3.4 [23–27]. These values reflect the overall (or molecular)
reactivity of the substrates regardless of the site or mechanism of the reaction.
Mechanistic studies have shown that the attack of the t-BuO: radical on Et3SiH
occurs in about 80 % of the cases at the SiH moiety and in 20 % at the ethyl groups
at 27 8C [23], whereas the attack on (Me3Si) 3SiH occurs in about 95 % of the cases
at the SiH moiety and in 5 % at the trimethylsilyl groups at 27 8C [26].
The rate constants for reaction of t-BuO: radical with silanes increase along
the series Et3SiH < Ph3SiH < (MeS) 3SiH < (Me3Si) 3SiH with the expected
intermediate values for silanes having mixed substituents. In particular, the rate
constants decrease along the two series PhSiH3 < Ph2SiH 2 < Ph3SiH and
Me3SiH < Me3SiSi(H)Me2 < (Me3Si) 2Si(H)Me < (Me3Si) 3SiH.
The Arrhenius parameters obtained for the reaction of t-BuO: radical with
Et3SiH are log A=M 1 s 1 ¼ 8:5 and Ea ¼ 8:8 kJ/mol [23]. The Arrhenius parameters
for Me3SiH in the gas phase are also available and were obtained by
competition with the tert-butoxyl radical decomposition, i.e., log A=M 1 s 1 ¼
8:7 and Ea ¼ 10:9 kJ/mol [28]. These preexponential factors lie in the expected
range and, therefore, the activation energy is expected to be the major factor
Table 3.4 Rate constants for reactions of tert-butoxyl [23–27] and cumylperoxyl [29]
radicals with silicon hydrides
Silane t-BuO:
k=M 1 s 1 at 27 8C a
n-C5H11SiH3 1:1 10 7
PhSiH3 7:5 10 6 0.90
Ph2SiH2 1:3 10 7
Et3SiH 5:7 10 6 0.10 c
PhMe2SiH 6:6 10 6 0.21
Ph2MeSiH 0.43
Ph3SiH 1:1 10 7 0.67
(Me3Si)Me2SiH 1:7 10 7
(Me3Si) 2MeSiH 6:2 10 7d
(Me3Si) 3SiH 1:1 10 8 66.3
H(PhSiH) nH 24.7 e
(MeS) 3SiH 4:4 10 7
(i-PrS) 3SiH 4:5 10 7
PhMe2COO:
k=M 1 s 1 at 73 8C b
a
Method: laser flash photolysis, if not otherwise mentioned. Solvent: 2:1 (v/v) di-tert-butyl peroxide/benzene or
1:4 (v/v) di-tert-butyl peroxide/isooctane.
b
Method: inhibited hydrocarbon oxidation. Solvent: cumene.
c
Referring to t-BuMe2SiH.
d
Average value of competitive studies (Me3Si)Me2SiH=(Me3Si) 2MeSiH and (Me3Si) 2MeSiH=(Me3Si) 3SiH in
tert-butylbenzene [6].
e
Value for each SiH moiety.